1. Field of the Invention
The present invention relates to an image formation apparatus which performs a density control operation.
2. Related Background Art
Conventionally, various methods such as an electrophotographic method, a thermal transfer method, an inkjet method and the like are applied as color image formation methods to a color image formation apparatus. In recent years, the image formation apparatus which applies the electrophotographic method has been widely used, because the electrophotographic method is excellent in the points of high speed performance, high image quality performance and silence performance as compared with other methods. It should be noted that the electrophotographic method includes various kinds of methods. Here, an example of a color image formation apparatus which applies a tandem method especially excellent in the high speed performance will be described.
FIG. 11 is a schematic diagram showing the color image formation apparatus which applies the tandem method. In the image formation apparatus of the tandem method, a toner image formation unit 8K for black, a toner image formation unit 8M for magenta, a toner image formation unit 8C for cyan and a toner image formation unit 8Y for yellow are independently disposed. An image-transferred material such as an ordinary sheet, an OHP (overhead projector) sheet or the like on which a toner image is to be transferred is carried by a carrying belt 7 which is extended by a driving roller 5 and subdriving roller 6, and passes the four toner image formation units sequentially. Every time the image-transferred material 1 passes the toner image formation unit, the toner image of the corresponding color is formed or overlaid on the material 1. Finally, a full-color image is formed on the image-transferred material 1.
Hereinafter, the color image formation apparatus which applies the tandem method will be explained in detail. Since the operations of the respective toner image formation units 8K, 8M, 8C and 8K are substantially the same, hereinafter the operation of only the toner image formation unit 8M will be described as an example of the representative. In the unit 8M, first, a photosensitive drum 9 which is rotatively driven in the direction indicated by the arrow is uniformly electrified at xe2x88x92600V by an electrifier 10 (hereinafter such a potential is called an electrification potential), and a latent image corresponding to a magenta image is formed on the drum 9 by a scan beam of a laser exposure optical system 11 or the like. A potential of the latent image formed by exposure with the scan beam is about xe2x88x92200V (hereinafter called an exposure portion potential). On the other hand, a magenta toner which has a certain quantity and was electrified to have a negative polarity is supplied onto a development roller 12, and a development bias is applied to the roller 12. Either a DC bias or a bias obtained by overlaying an AC bias on the DC bias can be used as the development bias. Even in case of either, by setting the DC component of the bias to have an appropriate value between the electrification potential and the exposure portion potential, it is possible to perform the development in which the toner is selectively adhered to the latent image on the photosensitive drum.
The magenta toner image thus formed on the photosensitive drum 9 is electrostatically transferred onto the image-transferred material 1 carried at the speed substantially the same as the rotation speed of the drum 9, by a positive-polarity transfer bias applied to a transfer roller 13.
Similarly, the above processing is performed in the toner image formation units 8C, 8Y and 8K respectively, whereby the toner image of four colors is formed on the image-transferred material 1. Next, the toner image is meltingly fixed to the image-transferred material 1 by a fixing unit 15, and the material 1 is then discharged from the apparatus.
On the other hand, in the color image formation apparatus, if the density of each color and a halftone gradation characteristic change due to change of available environment, change by long-term use and the like, a color tone of an output image changes. Thus, some image density control means have been often provided to prevent such a problem. Conventionally, in the image density control, a density control sequence described as follows is performed after a power supply is turned on, after a sleep (afterheat) state is released, after images of a certain number are output, and the like. Thus, it is possible to always obtain the stable output image. Hereinafter, an example of the image density control sequence will be described.
First, a toner image (i.e., a test patch) having a specific pattern is formed on the photosensitive drum 9 or the carrying belt, and the density of the formed patch is detected by a density sensor 20. As the test patch, a quadrangle pattern of 15 mmxc3x9715 mm is often used. As shown in FIG. 12, the density sensor 20 is mainly composed of a light emission element 21 such as an LED or the like, and a light reception element 22 such as a photodiode or the like. Thus, infrared light is irradiated on a pattern P by the light emission element 21, and the diffused light from the pattern P can be detected by the light reception element 22. The reason why the diffused light, rather than the regular reflection light, is used for the detection is that the diffused light is not influenced easily by the gap of an optical axis, the surface state of the background on which the test patch is formed, and the like. Since the reflection light detected by the light reception element 22 one-to-one correlates to the toner image density, the toner image density can be resultingly detected by the density sensor 20.
The image density is controlled on the basis of an image formation condition including the electrification potential of the photosensitive drum, the laser exposure quantity, the development bias and the like. The halftone gradation characteristic is controlled based on an image data conversion table. Thus, the plural patches are formed by stepwise changing such the image formation condition and the image data conversion table. The densities of these patches are detected by the density sensor 20, and the optimum value of the image formation condition is derived from the detected results.
As above, the density of the tentatively formed toner image is detected, and the detected result is fed back, thereby always obtaining the stable image.
Further, in the color image formation apparatus which applies the electrophotographic method, generally, there is the apparatus for which a low-speed fixing mode (or a low speed mode) is prepared to cope with the image-transferred material such as the OHP sheet, a board or the like. In the low speed fixing mode, since the fixing is performed at the speed lower than the ordinary fixing speed, a fixing time can be prolonged. Thus, even if the OHP sheet is used as the image-transferred material, the toner can be sufficiently melted, thereby increasing permeability. Besides, even if the board having a large heat capacity is used, thereby securing satisfactory fixability.
On the other hand, like the above color image formation apparatus, if the distance between the final transfer position (i.e., at the toner image formation unit 8K) and the nip position of the fixing roller is shorter than the length of the image-transferred material, it is impossible to lower the operation speed of only the fixing unit in the low speed mode. Namely, it is necessary to lower the entire speed (processing speed) of the image formation including carrying speed of the image-transferred material, rotation speed of the photosensitive drum, electrification speed, development speed and transfer speed.
Further, there is the color image formation apparatus for which a high resolution mode to decrease the processing speed is prepared so as to increase the image density in the circumference direction of the photosensitive drum. In the mode (i.e., an ordinary mode) of the ordinary processing speed, even if the optimum image formation condition according to the apparatus and its environment is previously determined based on the image density control sequence, such the image formation condition is not the optimum one in the low speed mode, whereby there is a problem that the quality of the image changes.
Further, it is possible in the low speed mode to additionally perform the density control sequence same as in the ordinary mode so as to settle or firm the formed image even in this low speed mode. However, in this case, the frequency of the control increases, and it takes time for the density control because the processing speed is slow, whereby there is a unfavorable problem that the time necessary for the user to wait for image printing seriously increases.
An object of the present invention is to provide an image formation apparatus and its control method by which the above-mentioned conventional problems can be solved.
Another object of the present invention is to provide an image formation apparatus and its control method which can obtain an image of proper quality without spending time even in a low speed mode.
Still another object of the present invention is to provide an image formation apparatus and its control method which determines an image formation control condition for a low speed mode, on the basis of an image formation control condition in ordinary processing speed.
Other objects and features of the present invention will become apparent from the following detailed description and the attached drawings.